Last year, the auction house sold in Hong Kong a 101.38-carat pear-shaped flawless diamond, dubbed The Key 10138, for more than $12 million in cryptocurrency.

The 1-billion-year-old Enigma diamond is believed to have come from outer space — either created from a meteoric impact or from a “diamond-bearing” asteroid that collided with Earth.

A natural faceted black diamond of this size is an “extremely rare occurrence,” according to Sotheby’s. The House of Gübelin in Switzerland and the Gemological Institute of America have both declared it the largest known of its kind, and it was first listed by Guinness World Records in 2006.

Its shape is also unique — it has been cut with 55 facets to represent the Middle-Eastern palm symbol of the Khamsa, which means five in Arabic, and stands for strength and protection.

Who discovered and shaped the gem is unknown and its previous owner chose to remain anonymous, Sotheby’s said.

Black diamonds, also known as Carbonado diamonds, are different from other coloured precious rocks because they do not get their shade from chemical impurities, such as nitrogen, hydrogen or boron — elements found in space.

Rather, black diamonds owe their colour to numerous dark inclusions (mostly graphite), and their opaqueness is caused by a “polycrystalline” structure that inhibits the reflection of light.

They also contain osbornite, a mineral present in meteorites.